Polyploid Oysters

Shells of Polyploid Oysters. Most organisms (including humans) have two sets of chromosomes; one from their mother, one from their father. A polyploid is an organism that has more than two sets of chromosomes. For oysters, this means having either three or four sets of chromosomes. These oysters are called triploid (3n) or tetraploid (4n), respectively.

How and why is a triploid oyster made?

Triploid (3n) oysters are produced by mating tetraploid (4n) males to diploid (2n) females. Triploids have become the animal of choice throughout Virginia. This is because of their proven superior growth and high survival, even under intense disease pressure. Can their performance be further improved with breeding? Testing has begun to determine if breeding better polyploids is possible.

Can polyploid oysters be improved?

The first step in this process is to see if and how much variation exists among triploid spawns produced from same tetraploid males and different females. This will reveal if the genetic advantage from a diploid female is carried into the triploid cross and thus if selection for an improved triploid is possible. ABC is also now making several new pure-bred tetraploid lines, and through new tetraploid family breeding, researchers can determine how much variation exists among triploid spawns produced from the same diploid female and different tetraploid males. Managing tetraploid populations represents an extensive program, which can be considered distinct from the ABC family and line breeding programs.

Stability of polyploidy oysters over time

ABC researchers have been systematically examining triploid and tetraploid populations of oysters for chromosome stability. Already known is that both tend to lose chromosomes over time. For triploids, this process is much slower than tetraploids, and ABC researchers have found that triploids do not lose a significant number of chromosomes before reaching market size. Does this chromosome loss affect the fertility of tetraploids? ABC research has shown that fertility in tetraploid oysters is largely unaffected, even after significant chromosome loss.